The present invention relates to the compression of digital data, and more particularly to a system for processing digitized video signals for transmission in a compressed form.
Television signals are conventionally transmitted in analog form according to various standards adopted by particular countries. For example, the United States has adopted the standards of the National Television System Committee ("NTSC"). Most European countries have adopted either PAL (Phase Alternating Line) or SECAM standards.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast via a cable television network or by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
A substantial amount of digital data must be transmitted in any digital television system. This is particularly true where high definition television ("HDTV") is provided. In a digital television system, a subscriber receives the digital data stream via a receiver/descrambler that provides video, audio, and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must be transmitted.
The video portion of a television signal comprises a sequence of video "frames" that together provide a moving picture. In digital television systems, each line of a video frame is defined by a sequence of digital data bits referred to as "pixels". A large amount of data is required to define each video frame of a television signal. For example, 7.4 megabits of data is required to provide one video frame at NTSC resolution. This assumes a 640 pixel by 480 line display is used with 8 bits of intensity value for each of the primary colors red, green, and blue. High definition television requires substantially more data to provide each video frame. In order to manage this amount of data, particularly for HDTV applications, the data must be compressed.
Video compression techniques enable the efficient transmission of digital video signals over conventional communication channels. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the important information in a video signal. The most powerful compression systems not only take advantage of spatial correlation, but can also utilize similarities among adjacent frames to further compact the data. In such systems, differential encoding is usually used to transmit only the difference between an actual frame and a prediction of the actual frame. The prediction is based on information derived from a previous frame of the same video sequence.
An example of a video compression system using motion compensation is described in Ninomiya and Ohtsuka, "A Motion-Compensated Interframe Coding System for Television Pictures," IEEE Transactions on Communications, Vol. COM-30, No. 1, January 1982. The motion estimation algorithm described therein is of the block-matching type. In this case, a motion vector is determined for each block in the current frame of an image by identifying a block in the previous frame which most closely resembles the particular block. The entire current frame can then be reconstructed at a decoder by sending the difference between the corresponding block pairs, together with the motion vectors that are required to identify the corresponding pairs. Often, the amount of transmitted data is further reduced by compressing both the displaced block differences and the motion vector signals. Block matching motion estimation algorithms are particularly effective when combined with block-based spatial compression techniques such as the discrete cosine transform (DCT).
Other examples of motion compensation systems can be found in U.S. Pat. No. 4,802,006 to Iinuma, et al., entitled "Signal Processing Unit for Producing a Selected One of Signals Predictive of Original Signals," U.S. Pat. No. 4,816,906 to Kummerfeldt, et al., entitled "Method for Motion-Compensated Frame-to-Frame Prediction Coding," U.S. Pat. No. 4,827,340 to Pirsch, entitled "Video-Signal DPCM Coder with Adaptive Prediction," U.S. Pat. No. 4,897,720 to Wu, et al., entitled "Circuit Implementation of Block Matching Algorithm," and European patent publication no. 0 237 989 to Takenaka, et al., entitled "Differential Coding Apparatus Having an Optimum Predicted Value Determining Circuit." In the '340 patent, adaptive differential pulse code modulation (DPCM) switching is effected on a block-by-block basis between different predictors, such as a two-dimensional intraframe predictor and a pure interframe predictor. The block sizes of the different predictors is the same.
Like most other motion estimation algorithms, the performance of the block-matching method is dependent on how well the movement from one frame to the next can be modeled as a simple translation. In television applications, movements may involve zooming, rotation, and many other complex distortions that cannot be accurately modeled as a simple translation. In such cases, compression artifacts are more likely to become visible since the accuracy of the prediction is reduced.
It would be advantageous to provide apparatus for improving the performance of motion compensated video signal compression systems, particularly when complex movements occur. The present invention provides such apparatus.